Charles Darwin

“ It is a much more remarkable fact that when an object, such as a bit of meat or an insect, is placed on the disc of a leaf, as soon as the surrounding tentacles become considerably inflected, their glands pour forth an increased amount of secretion. I ascertained this by selecting leaves with equal-sized drops on the two sides, and by placing bits of meat on one side of the disc; and as soon as the tentacles on this side became much inflected, but before the glands touched the meat, the drops of secretion became larger. ”

“ As the process of aggregation can be excited by a few touches, or by the pressure of insoluble particles, it is evidently independent of the absorption of any matter, and must be of a molecular nature. ”

“ It is surprising how minute a particle of any substance, such as a bit of thread or hair or splinter of glass, if placed in actual contact with the surface of a gland, suffices to cause the tentacle to bend. If the object, which has been carried by this movement to the centre, be not very small, or if it contains soluble nitrogenous matter, it acts on the central glands; and these transmit a motor impulse to the exterior tentacles, causing them to bend inwards. ”

“ We may, therefore, conclude that the ferment of Drosera is closely analogous to, or identical with, the pepsin of animals.
The substances which are digested by Drosera act on the leaves very differently. Some cause much more energetic and rapid inflection of the tentacles, and keep them inflected for a much longer time, than do others. We are thus led to believe that the former are more nutritious than the latter, as is known to be the case with some of these same substances when given to animals; for instance, meat in comparison with gelatine. ”

“ Therefore it is a very interesting spectacle to watch the effects on the cells of a gland, of the pressure of a bit of hair, weighing only 1/78700 of a grain and largely supported by the dense secretion, for this excessively slight pressure soon causes a visible change in the protoplasm, which change is transmitted down the whole length of the tentacle, giving it at last a mottled appearance, distinguishable even by the naked eye. ”

“ It might, however, happen, under certain conditions, that a plant, after having acquired the power of digestion, should degenerate into one capable only of absorbing animal matter in solution, or in a state of decay, or the final products of decay, namely the salts of ammonia. It would appear that this has actually occurred to a partial extent with the leaves of Aldrovanda; the outer parts of which possess absorbent organs, but no glands fitted for the secretion of any digestive fluid, these being confined to the inner parts. ”

“ Aggregation may be seen to occur very quickly if a piece of a leaf is immersed in a weak solution of carbonate of ammonia. Again, small cubes of albumen and gelatine were left for eight days on a leaf, which was then cut open. The whole surface was bathed with acid secretion, and every cell in the many glands which were examined had its contents aggregated in a beautiful manner into dark or pale purple, or colourless globular masses of protoplasm. These underwent incessant slow changes of forms; sometimes separating from one another and then reuniting, exactly as in the cells of Drosera. ”

“ Occasionally a gland, with its light load, could be seen through a strong lens to move an extremely short distance and then stop; this was especially apt to occur when excessively minute particles, much less than those of which the measurements will be immediately given, were placed on glands ; so that we here have nearly the limit of any action.
I was so much surprised at the smallness of the particles which caused the tentacles to become greatly inflected that it seemed worth while carefully to ascertain how minute a particle would plainly act. ”

“ One leaf of the Pinguicula had caught ten of the little leaves of the Erica; and three leaves on the same plant had each caught a seed. Seeds subjected to the action of the secretion are sometimes killed, or the seedlings injured. We may, therefore, conclude that Pinguicula vulgaris, with its small roots, is not only supported to a large extent by the extraordinary number of insects which it habitually captures, but likewise draws some nourishment from the pollen, leaves, and seeds of other plants which often adhere to its leaves. ”

“ A minute quantity of a weak solution produces no effect; add more, or give a rather stronger solution, and the tentacles bend. Touch a gland once or twice, and no movement follows; touch it three or four times, and the tentacle becomes inflected. But the nature of the substance which is given is a very important element: if equal-sized particles of glass (which acts only mechanically) , of gelatine, and raw meat, are placed on the discs of several leaves, the meat causes far more rapid, energetic, and widely extended movement than the two former substances. ”

“ When a dog stands a quarter of a mile to leeward of a deer or other animal, and perceives its presence, the odorous particles produce some change in the olfactory nerves; yet these particles must be infinitely smaller* than those of the phosphate of ammonia weighing the one-twenty-millionth of a grain. These nerves then transmit some influence to the brain of the dog, which leads to action on its part. With Drosera, the really marvellous fact is, that a plant without any specialised nervous system should be affected by such minute particles ”

“ Even if an insect alights on a leaf, a slight molecular change is probably transmitted to some distance across its tissue, with the sole difference that no perceptible effect is produced. We have some evidence in favour of this belief, for we know that a single touch on the glands of Drosera does not excite inflection; yet it must produce some effect, for if the glands have been immersed in a solution of camphor, inflection follows within a shorter time than would have followed from the effects of camphor alone. ”

“ We know from the strong odour of cooked cabbage leaves that boiling-water produces some chemical change in them, and that they are thus rendered far more digestible and nutritious to man. It is therefore an interesting fact that water at this temperature extracts matter from them which excites Drosera to an extraordinary degree.
Grasses contain far less nitrogenous matter than do peas or cabbages. The leaves and stalks of three common kinds were chopped and boiled for some time in distilled water. ”

“ We learn from the foregoing experiments that the margins of the leaves curl inwards when excited by the mere pressure of objects not yielding any soluble matter, by objects yielding such matter, and by some fluids—namely an infusion of raw meat and a week solution of carbonate of ammonia. A stronger solution of two grains of this salt to an ounce of water, though exciting copious secretion, paralyses the leaf. Drops of water and of a solution of sugar or gum did not cause any movement. ”

“ It is surprising how slightly damp a bit of meat or albumen need be in order to excite secretion and afterwards slow movement, and equally surprising how minute a quantity of animal matter, when absorbed, suffices to produce these two effects. It seems hardly credible, and yet it is certainly a fact, that a bit of hard-boiled white of egg, first thoroughly dried, then soaked for some minutes in water and rolled on blotting paper, should yield in a few hours enough animal matter to the glands to cause them to secrete, and afterwards the lobes to close. ”

“ When the stomach of an animal is mechanically irritated, it secretes an acid, and when particles of glass or other such objects were placed on the glands of Drosera, the secretion, and that of the surrounding and untouched glands, was increased in quantity and became acid. But, according to Schiff, the stomach of an animal does not secrete its proper ferment, pepsin, until certain substances, which he calls peptogenes, are absorbed; and it appears from my experiments that some matter must be absorbed by the glands of Drosera before they secrete their proper ferment. ”

“ As we have seen that nitrogenous fluids act very differently on the leaves of Drosera from non-nitrogenous fluids, and as the leaves remain clasped for a much longer time over various organic bodies than over inorganic bodies, such as bits of glass, cinder, wood, &c., it becomes an interesting inquiry, whether they can only absorb matter already in solution, or render it soluble,—that is, have the power of digestion. ”

“ The glands apparently are acted on more strongly by a solution of urea, and less strongly by an infusion of raw meat, than are the processes. The case of urea is particularly interesting, because we have seen that it produces no effect on Drosera, the leaves of which are adapted to digest fresh animal matter. But the most important fact of all is, that in the present and following species the quadrifid and bifid processes of bladders containing decayed animals generally include little masses of spontaneously moving protoplasm; whilst such masses are never seen in perfectly clean bladders. ”

“ Finally, as so many and such widely different causes excite aggregation, it would appear that the living matter within the gland-cells is in so unstable a condition that almost any disturbance suffices to change its molecular nature, as in the case of certain chemical compounds. And this change in the glands, whether excited directly, or indirectly by a stimulus received from other glands, is transmitted from cell to cell, causing granules of protoplasm either to be actually generated in the previously limpid fluid or to coalesce and thus to become visible. ”

“ It seems at first sight a probable view that aggregation is due to the glands being excited to secrete more copiously, so that sufficient fluid is not left in their cells, and in the cells of the pedicels, to hold the protoplasm in solution. In favour of this view is the fact that aggregation follows the inflection of the tentacles, and during the movement the glands generally, or, as I believe, always, secrete more copiously than they did before. ”

“ The absorption of animal matter from captured insects explains how Drosera can flourish in extremely poor peaty soil, in some cases where nothing but sphagnum moss grows, and mosses depend altogether on the atmosphere for their nourishment. Although the leaves at a hasty glance do not appear green, owing to the purple colour of the tentacles, yet the upper and lower surfaces of the blade, the pedicels of the central tentacles, and the petioles contain chlorophyll, so that, no doubt, the plant obtains and assimilates carbonic acid from the air. ”

“ If a cell is ruptured, neither the exuded matter nor that which still remains within the cell undergoes aggregation when carbonate of ammonia is added. A very strong solution of this salt and rather large bits of raw meat prevent the aggregated masses being well developed. From these facts we may conclude that the protoplasmic fluid within a cell does not become aggregated unless it be in a living state, and only imperfectly if the cell has been injured. ”

“ A living insect is a more efficient object than a dead one, as in struggling it presses against the glands of many tentacles. An insect, such as a fly, with thin integuments, through which animal matter in solution can readily pass into the surrounding dense secretion, is more efficient in causing prolonged inflection than an insect with a thick coat, such as a beetle. ”

“ Aggregation is excited by almost all the stimulants which induce movement; such as the glands being touched two or three times, the pressure of minute inorganic particles, the absorption of various fluids, even long immersion in distilled water, exosmose, and heat. Of the many stimulants tried, carbonate of ammonia is the most energetic and acts the quickest: a dose of 1/134400 of a grain (.00048 mg.) given to a single gland suffices to cause in one hour well-marked aggregation in the upper cells of the tentacle. ”